A conventional motor drive device, of which limiter function increases a limit value of the limiter step by step at given time intervals from the start of driving, has been proposed in, e.g. Patent Literature 1. This motor drive device changes a duty ratio of a drive pulse used in PWM (Pulse Width Modulation) driving method, and drives the motor with a driving electric power in response to this duty ratio. The duty ratio starts from a predetermined one which gives the minimum power to drive a rotor when the motor starts driving, and then the duty ratio increases step by step at given time intervals with the motor kept driving. The foregoing mechanism allows the conventional drive device to drive the motor in a soft starting manner, and to reduce useless electric current at the start.
The foregoing conventional motor drive device intends to reduce the electric current needed at the start of the motor; however, a large amount of current, e.g. rush current, flows when a command value is changed, for instance, a speed of the motor is changed. The reduction in the electric current at the start of the motor is thus not enough for reducing the electric current in total. FIG. 20 shows changes in electric current when the speed of the conventional motor is changed. In FIG. 20, the waveforms on the upper part of the chart indicate a speed command that gives instruction of speed, and an actual speed of the motor in response to the instruction. The waveform on the middle part of the chart indicates a change in duty ratio of a PWM signal. The waveform on the lower part of the chart indicates a change in a motor current when the speed is changed as shown in the waveform on the upper part of the chart. As FIG. 20 shows, when the motor speed is changed to a higher one, a large amount of current such as a rush current flows.
As shown in FIG. 20, setting a current threshold amount (Ith) allows preventing the foregoing rush current; however, a current limitation works instantaneously in this case, so that the motor generates noises or encounters a great shock to its mechanical section when the electric current is limited.
In a case when the motor speed is lowered, a regeneration phenomenon of the motor inversely supplies a regeneration power to the motor drive device, so that a power supply voltage rises, thereby sometimes malfunctioning the devices connected to the power supply line.    Patent Literature 1: Unexamined Japanese Patent Application Publication No. 2000-116178